Slave station with two output circuits commonly and directly connected to a line for serially transmitting data to a master station in two operational modes

ABSTRACT

The communications system operates at two distinct operating modes. Each of the slave stations, which are controlled by a master station, has a first output circuit and a second output circuit. The two output circuits are connected to one another on the output side. In a first operating mode, the slave stations are identified by the master station, all the first output circuits can be activated and the system operates at a low clock rate. In a second operating mode, a transfer of data takes place, one of the second output circuits can be activated, and the system operates at a higher clock rate.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of international application Ser. No. PCT/DE97/00704, filed Apr. 7, 1997.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a communications system which has at least one slave station controlled by a master station. The slave station(s) here cannot operate without the master station.

A communications system can be realized, for example, by means of the I²C (Inter IC) bus. A plurality of master stations and a plurality of slave stations are connected to the I²C bus via open-collector outputs. The master stations control the slave stations using addresses which are known to them and which are permanently stored in the slave stations.

It is possible, in the case of the I²C bus, for two master stations simultaneously to attempt to access the bus in order to address slave stations. For this case, a so-called arbitration procedure is provided for the I²C bus. In this procedure, the competing master stations simultaneously transmit bit by bit in each case one individual identification code, stored in them, into a precharged line of the bus via their open-collector outputs. The potential of the line is drawn to ground as soon as one of the bits which is about to be output has a high level, even if the bit of the other master station has a low level. Each of the master stations monitors whether that bit of its identification code which has just been output has a high level when the line is being discharged. If this is not the case, the master station deactivates itself and that master station whose identification code with all its bits is decisive for the successive potential states of the line manages to access the bus.

A disadvantage of the I²C bus is that the maximum data rate is limited by the exclusive use of open-collector outputs.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a communications system with a master unit and at least one slave unit controlled by the master unit, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type in which, on the one hand, it is possible to connect any desired number of slave stations whose addresses and whose number are not known beforehand to the master station controlling them, and in which, on the other hand, the maximum data rate is improved in comparison with that of the I²C bus.

With the foregoing and other objects in view there is provided, in accordance with the invention, a communications system, comprising:

a master station, at least one slave station, and at least one line connecting the master and slave stations;

the at least one slave station including a first output circuit and a second output circuit each having an output commonly connected to the at least one line;

the first output circuits of the at least one slave station, in a first operating mode of the system, outputting an identification code bit by bit to the line; and.

in a second operating mode, the master station addressing the at least one slave station and the second output circuit of the slave station transmitting different data to the master station through the line.

In accordance with an added feature of the invention, the at least one slave station is one of a plurality of slave stations and, wherein, in the first operating mode, the output circuits of the plurality of slave stations output identification codes simultaneously bit by bit to the line.

In accordance with an additional feature of the invention, in the second operating mode, the master station addresses a selected one of the slave stations and only the respectively addressed slave station transmits different data to the master station.

In accordance with another feature of the invention, the master station contains a clock generator and the clock signal is transmitted through a clock line to the at least one slave station. The clock signal synchronizes the operation of the master station and the slave stations; wherein a clock rate of the clock signal is lower in the first operating mode than in the second operating mode.

In accordance with a further feature of the invention, connecting devices are provided for connecting the slave stations. The connecting devices include detectors for detecting whether a respective slave station is connected and for transmitting a corresponding signal to the master station.

With the above and other objects in view there is also provided, in accordance with another feature of the invention, a slave station for a communications system operable in two distinct operating modes, comprising:

a first output circuit with an output outputting an identification code bit by bit in a first operating mode of the communications system; and

a second output circuit with an output outputting data different from the identification code in a second operating mode of the communications system;

wherein the outputs of the first and second output circuits are connected to one another.

In accordance with again a further feature of the invention, the first output circuit assumes a state with a relatively higher impedance when a datum of a first logic state is output and a state with a relatively lower impedance when a datum of a second logic state is output.

The invention permits the slave stations to be identified by the master station in a first operating mode with low system clock rate by using the first output circuits and a data transfer in a second operating mode with a high clock rate by using the second output circuits.

In the first operating mode, the first output circuits can be operated in parallel in a “wired-or” connection and permit the slave stations to be identified by the master station. This is achieved in that the first output circuit assumes a state with higher impedance when a datum of a first logic state is output than when a datum of a second logic state is output. A first output circuit of this type is for example an open-drain or open-collector output circuit which, however, has a relatively low switching speed in comparison with tristate output circuits.

In accordance with a concomitant feature of the invention, the second output circuit is a tristate output circuit. With such tristate outputs it is possible to achieve higher data rates in the second operating mode, in which the system is usually in, than with the I²C bus.

Other features which are considered as characteristic for the invention are set forth in the appended claims.

Although the invention is illustrated and described herein as embodied in a communications system with a master station and at least one slave station, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a first exemplary embodiment of the communications system according to the invention;

FIG. 2 is a schematic view of an exemplary embodiment of the master station of FIG. 1;

FIG. 3 is a schematic view of an exemplary embodiment of the slave stations of FIG. 1;

FIG. 4 is a schematic view of a second exemplary embodiment of the communications system according to the invention;

FIG. 5 is a schematic view of an exemplary embodiment of the slave stations of FIG. 4; and

FIGS. 6 and 7 are flow charts illustrating an operating method for the communications systems of FIGS. 1 and 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 which illustrates a first exemplary embodiment of the communications system, there is seen a bus which has a clock line CLK and a command and data line C/D. Other lines which are necessary for transmitting supply potentials are not illustrated. Two slave stations S and one master station M are connected to the clock line CLK and to the command and data line C/D, the master station M being used to control the slave stations S.

FIG. 2 illustrates the layout of the master station M of FIG. 1. The master station includes a clock generator CLKG which can be connected to the clock line CLK and which generates a common operational clock signal for the master station M and the slave stations S. The operational clock permits synchronous operation of the stations M and S.

Furthermore, the master station M has further elements CTRM which are connected to the command and data line C/D via an output circuit OUTM and an input circuit INM. The output circuit OUTM is used to output commands and data to be stored in the slave stations, while the input circuit INM is used to receive command confirmations of the slave stations S and data transmitted by said slave stations S.

The master station M also has an address-generator ADRG by means of which it can assign, during an identification process which will be explained later, addresses for later addressing to the slave stations S. It is thereby possible to transmit the addresses to the slave stations S likewise via the output circuit OUTM and the command and data line C/D in certain circumstances, as will be described below.

The command and data line C/D is connected through a resistor R (in this case a pull-up resistor) to a first potential VCC which may be a supply potential of the master station M. In this exemplary embodiment of the invention, the resistor R is arranged within the master station M. It can however also be arranged outside the latter. The resistor R can be deactivated with a switch S.

Each of the slave stations S of the communications system in FIG. 1 accords to FIG. 3. Each slave station S is connected to the command and data line C/D via a first output circuit OC (an open-drain output circuit) and a second output circuit TR (a tristate output circuit for a push-pull mode). The first and second output circuits OC, TR are connected to one another on the output side. The second output circuit TR has a first transistor T1 and a second transistor T2. The first output circuit OC has a third transistor T3. The first output circuit OC can, of course, also be realized as an open-collector output with a bipolar transistor.

The slave station S in FIG. 3 has components CTRS to which the operational clock generated by the master station M can be fed via the clock line CLK. The slave station S is connected to the command and data line C/D via an input circuit INS. The input circuit INS is used to receive an address which is assigned to the slave station S by the master station M, and commands and data which can be transmitted by the master station M.

In the slave station S, an identification code ID is stored which is different for all the slave stations S connected to the communications system. The identification code ID can be output bit by bit to the command and data line C/D via the first output circuit OC. This can take place simultaneously for all the slave stations S, specifically in synchronism with the operational clock CALK.

The slave station S also has a memory REG1 (a RAM) which can be written to and in which an address transmitted by the master station M can be stored. In addition, the slave station S has a read-only memory REG2 (a ROM) in which an initialization address which is identical for all the slave stations S is permanently stored. Furthermore, the slave station S contains a monitoring means U by means of which, when the identification code ID is output via the first output circuit OC, the individual bits of the identification code ID can be compared with the potential of the command and data line C/D. The slave station S also contains a data memory MEM in which data transferred via the command and data line C/D can be stored and can be output again via the second output circuit TR. The data memory MEM can be a RAM or a ROM.

The method of operation of the communications system components described above with reference to FIGS. 1 to 3 will now be explained below.

The communications system, and with it the slave stations S, can be placed in two different operating modes by the master station M. In the first operating mode, the slave stations S which are connected to the communications system are identified by the master station M which assigns to the identified slave stations S addresses for later individual addressing. In the second operating mode, individual slave stations S can be addressed by the master station M using these addresses and the data which is stored, or is to be stored, in the data memory MEM is transferred between the addressed slave station S and the master station M.

The second output circuits OC of all the slave stations S can be activated in the first operating mode. In the second operating mode, however, only the second output circuit TR of the individual slave stations S which are respectively addressed can be activated.

When the communications system is activated, all the slave stations S are firstly placed in the first operating mode. In response to a command of the master station M, which addresses all the slave stations S using the initialization address which is stored in the read-only memory REG2 and is identical for all the slave stations S, all the slave stations S simultaneously transmit bit by bit their identification codes, via their first output circuit OC, to the command and data line C/D which is firstly precharged to the first potential VCC.

The first output circuits OC are in a state with higher impedance (third transistor T3 is disabled) when an item of data of a first logic state 0 is output than when an item of data of a second logic state 1 is output (third transistor T3 is conductive). Since the command and data line C/D is precharged to the first potential VCC, this first potential VCC is maintained on the line C/D, provided the first logic state 0 is present at all the first output circuits OC. If, in contrast, the second logic state 1 is present at the first output circuit OC at only one of the slave stations S, the potential of the line C/D is discharged to ground via the corresponding third transistor T3 until only bits of the first logic state 0 are present again at the first output circuits OC.

While all the slave stations S output their identification codes ID, they monitor the potential of the line C/D by means of their monitoring means U. Those slave stations S which output bits of the first logic state 0, but detect that the line C/D is nevertheless discharged, deactivate themselves. Then only those slave stations S which are currently outputting a bit of the second logic state 1 remain active. After all the bits of the identification codes ID have been output, there is consequently only one of the slave stations S still active, since all the identification codes ID are different.

Since the number of bits of the identification codes ID is known and the bit-by-bit outputting takes place in synchronism with the operational clock CALK, the master station M knows when all the bits have been output and, at this time, it assigns an address for later addressing to the last slave station S still active by means of its address-generator ADRG. There are two possibilities for this address assignment:

1. The master station M generates an individual address by means of its address-generator ADRG and transmits that address via the command and data line C/D to the still active slave station S where it is stored in the slave station's programmable memory REG1.

2. As an alternative to this, the master station M observes, during the identification process (outputting of the identification codes ID) just described, the potential on the line C/D and reconstructs from it the identification code of the last slave station S still active, which identification code it stores in the address-generator ADRG for later addressing of the slave station S using this identification code ID. The address which is assigned to the slave station S is then identical to its identification code ID determined by the master station M. The advantage of this variant consists in the fact that a programmable memory REG1 is not required in the slave stations S nor is there any need for transfer of an address which has been newly assigned to said stations.

The first-mentioned alternative provides the major advantage that the slave station S can be assigned a new address which has substantially fewer bits than its identification code ID. Also, the master station M does not need to have any means for detecting the identification codes ID output by the slave stations S. Since all the identification codes ID are to be different and they are stored in the read-only memory REG2, for example during the process of manufacturing the slave stations S, while the later place of use is still unknown, a large number of fabricated slave stations S require the provision of a large number of bits of the identification codes ID. On the other hand, in a communications system there will always be a limited number of subscribers, so that individual addressing of all the slave stations S is possible even with a small number of bits. The advantage of a smaller number of address bits is that the addressing procedures, which take the form of the transfer of address bits from the master station M to the slave stations S in the second operating mode, can be substantially shortened. There may be provision, for example, for the identification codes ID each to have 128 bits and for the assigned addresses to have only 32 bits.

In order to be able to identify more than one of the slave stations S and to be able to assign an address to each of them, the method which has been explained is repeated in connection with the slave stations S which have not yet been identified, while the slave stations S which have already been identified remain deactivated. In this way, after n identification cycles—during which in each case the identification codes ID are output bit by bit in the manner described—n slave stations S can be identified.

By virtue of the fact that, in order to carry out the first operating mode—the identification of the slave stations S by the master station—the first output circuits OC are provided as open-drain output circuits, there arises the favorable possibility of connecting all the slave stations S in parallel, as is necessary for the identification according to the invention. However, the switching behavior of such open-drain outputs is relatively slow. The use of the second output circuit TR as a tristate output, which can be switched more quickly in comparison with the latter, in order to carry out the second operating mode—the transfer of data from one of the slave stations S to the master station M—advantageously permits a much higher data rate than would be possible using the first output circuit OC, also in the second operating mode.

In order to set the two different data rates, the clock which is generated on the clock line CLK by the clock generator CLKG of the master station M can be set to two different values, namely a lower clock rate for the first operating mode in which the first output circuits OC are actuated, and a higher clock rate for the second operating mode in which the data stored in the data memories MEM can be transmitted via the corresponding second output circuits TR.

The command and data line C/D in FIG. 1 is used:

in the first operating mode (identification), to transfer the identification command of the master station M to all the slave stations S addressed via the initialization address stored in the read-only memory REG2, to output the identification codes ID of the slave stations S via the first output circuits OC and, if appropriate, to transmit the assigned addresses from the master station M to the respective slave station S,

in the second operating mode (data transfer), to transfer commands of the master station M to individual slave stations S by means of the newly assigned addresses stored in the memory REG1 which can be written to, to transmit data which can be, or is, stored in the addressed slave stations S, between the master station M and the respectively addressed slave station S as well as to transfer possibly provided command confirmations by the slave stations S as a response to commands of the master station M.

By virtue of using the second output circuits TR in the second operating mode, the transfer of data as well as the confirmation of the commands can be effected at a higher data rate than if the first output circuits OC were also used for this. This follows from the higher data rate which can be reached for tristate output circuits in comparison with open-drain output circuits. This is achieved in that the operational clock on the clock line CLK is increased in the second operating mode in comparison with the first operating mode.

It is particularly advantageous that the resistor R for precharging the line C/D (see FIG. 2) can be deactivated via the switch S in the second operating mode. The switch S is closed only in the first operating mode, since the precharging is necessary only for the parallel operation of the first output circuits OC. In contrast, the second output circuits TR can be operated at a maximum data rate if the switch S is open in the second operating mode.

FIG. 4 shows a second exemplary embodiment of the communications system according to the invention. The latter has a bus comprising three lines: the clock line CLK as well as a command line C and a data line D which are provided instead of the combined command and data line C/D from FIG. 1. Data stored in the data memory MEM of the slave stations S is transmitted via the data line D, while the commands and command confirmations are exchanged between the master station M and the slave stations S via the command line C. Furthermore, the identification according to the invention takes place via the command line C.

The command line C can be precharged to the first potential VCC via a current source I. The current source I may be, by way of example, a transistor and it may be deactivated with a switch S. With such a current source I, the discharging procedure of the command line C can be made quicker in the first operating mode (identification) than when a resistor R, as illustrated in FIG. 1, is used. The current source I can, of course, also be a component of the master station M. The possibility of deactivating the current source I, as is described in connection with the resistor R in FIG. 2, enables the recharging times of the line C in the second operating mode (use of the second output circuits TR) to be reduced, so that the maximum data rate which can be achieved with the second output circuits TR is increased further.

The master station M in FIG. 4 may be similar to that in FIG. 2. It is, of course, necessary to provide separate connections for the data line D and the command line C. The transfer of the assigned addresses to the slave stations S is effected via the command line C.

FIG. 5 shows one of the slave stations S from FIG. 4 which differs from the exemplary embodiment illustrated in FIG. 3 only in terms of the following points:

For connection to the data line D there is an input and output circuit I/O via which data can be read into and out of the data memory MEM. Appropriately, the input and output circuit I/O can also have a tristate output for ensuring a high data rate.

The first OC and second TR output circuits of the slave stations S are—as in the exemplary embodiment according to FIG. 3—again open-drain or tristate output circuits and are connected on the output side to the command line C. The first output circuit OC is, in this exemplary embodiment, part of the second output circuit TR which is formed by the first transistor T1 and the second transistor T2. The first output circuit OC is formed by the second transistor T2 of the second output circuit TR. The second output circuit TR can be controlled via the gates of the two transistors T1, T2 and, in this way, it can transmit one of the two logic states 0, 1 at its output or switch the latter to a high-impedance state. On the other hand, the first output circuit can be controlled via the gate of the second transistor T2, provided that the control signal of the first transistor T1 remains deactivated, so that the latter becomes disabled.

In the communications system illustrated in FIG. 4, the command line C is used exclusively to transfer commands of the master station M to the slave stations S, and command confirmations in the opposite direction. The inventive switching over from operating the first output circuits OC in the first operating mode (identification) to operating the second output circuits TR in the second operating mode (transfer of data) permits the commands received by the slave stations S to be confirmed at a high data rate in the second operating mode. The reason for this is, as already mentioned, the higher switching speed which can be reached by tristate output circuits in comparison with open-rain/collector output circuits. Since the communications system, and thus also the slave stations S, operate in synchronism with the operational clock CLK, the data rate of all the output circuits OC, TR, I/O is likewise determined by the clock CLK.

Overall, with this embodiment of the invention, as also in the exemplary embodiment according to FIG. 1, a high data rate for operation in the second operating mode is obtained, since, in the second operating mode, a higher rate of the clock on the clock line CLK is provided than in the first operating mode. The clock CLK is in fact used in FIG. 1 to synchronize the operation of the command and data line C/D and is used in FIG. 4 to synchronize the operation both of the command line C and of the data line D.

Since the identification in the first operating mode always takes only a relatively short time, the communications system is almost always in the second operating mode and is able to profit from the higher clock rate which the use of tristate output circuits both as second output circuits TR and within the input and output circuit I/O (in the exemplary embodiment in FIG. 5) makes possible.

FIG. 4 additionally shows as an example of the slave station S illustrated in the upper region that the latter is connected via a connecting device A to the lines CLK, C, D of the communications system. The connecting device A can be for example a module slot for the slave station S. It is then possible for the system to have a series of such connecting devices A, not all of which have to be connected to slave stations S when the system is operating. The connecting device A in FIG. 4 has a detection means DM which is used to detect the connection of the slave station S to the connecting device A. This detection can be realized for example by means of a mechanical or electric switch which is activated when the slave station S is connected to the connecting device A. After it has detected the connection of a slave station S, the detection means DM transfers an appropriate result signal to the master station M. In the exemplary embodiment shown, this result signal is transmitted via the command line C.

The described detection means DM makes it possible to inform the master station M if new slave stations S, which have previously not been identified or provided with an address, are connected to the communications system. This is particularly favorable if the number of connected slave stations S increases while the communications system is operating as a result of the addition of further slave stations S. The master station M can then deactivate the already previously identified slave stations S, after which the one or more newly added slave stations S which have not yet been assigned an address by the master station M are identified in the manner already described.

An advantageous mode of operating the communications systems shown in FIGS. 1 and 4 will now be explained with reference to an exemplary embodiment and to FIGS. 6 and 7:

As indicated at the very top of FIG. 6, when the communications system is activated, that is to say when the supply voltage is applied (power-on), all the slave stations S are initially in an idle state. All the slave stations S can be placed in this idle state at any time by a reset command CMD0 of the master station M while the communications system is operating.

When the communications system is activated, the slave stations S can be addressed by the master station M using the address which is stored in the read-only memory REG2 and is identical for all slave stations S. By means of a first command CMD1, the master station M places the slave stations S in a ready state in which the open-drain output circuits OC are ready for operation.

A second command CMD2 of the master station M (the identification command) places the slave stations S in an identification state which corresponds to the first operating mode described above. In this identification state, the bit-by-bit outputting of the identification codes ID is effected via the first output circuits OC.

An assigned address is transferred by means of a third command CMD3 to that slave station S which prevailed during the identification and it is subsequently placed in a stand-by state. In this stand-by state it no longer reacts to the commands CMD2 and CMD3.

After this one identification cycle has been carried out, the slave stations S which have not yet been identified are in the ready state again, after which the identification of the next slave station S is initiated by means of the second command CMD2.

When all the slave stations S have been identified, they can be changed by means of a fourth command CMD4 from the stand-by state to a state (push-pull state) in which the open-drain output circuits OC are deactivated and the tristate output circuits TR are held ready for operation.

If an additional slave station S is connected to the communications system, the master station M can be informed of this, as has been explained in connection with FIG. 4, by means of the detector DM. The master station M can return the slave stations S which are in the push-pull state and which, of course, have already been identified, to the stand-by state by means of a fifth command CMD5. The at least one slave station S, which has been newly connected to the system, is in the idle state as the result of it being supplied with the supply voltage for the first time. Using the commands CMD1, CMD2 and CMD3, the identification procedure already described can be carried out for this slave station or stations S.

FIG. 7 illustrates the already explained idle state, push-pull state and stand-by state of the slave stations S. The further operating states of the communications system will now be explained.

Starting from the push-pull state, in each case one of the slave stations S can be placed in a transfer state by means of the individual address which is assigned to it by the master station M in the identification state by means of the third command CMD3. At this time, a point-to-point connection is set up between this slave station S and the master station M. In this transfer state, the slave station S confirms (acknowledges) every command of the master station M. Using a ninth command CMD9, the slave station S is requested by the master station M to transfer to the master station M specific data stored in the slave station S. The specific data may be for example the capacity of the data memory MEM, the type of error correction code to be used for the data stored in the data memory MEM as well as the maximum possible clock rate which is acceptable for the operational clock for operating the slave circuit S.

By means of an eleventh command CMD11, the master station M can request the addressed slave station S which is in the transfer state to transfer to it data which is stored in the data memory MEM via the command and data line C/D (FIG. 1) and via the data line D, respectively, (FIG. 4). With the eleventh command CMD11, start address is transferred within the data memory MEM to the slave station S starting from which data is to be read out. The transfer of data takes place until the slave station S receives a stop command CMD12 or renews the eleventh command CMD11 which informs it of a new start address for reading data out of the data memory MEM.

The fifth command CMD5 is used not only to move slave stations S which are in the push-pull state into the stand-by state, as explained with reference to FIG. 6, but also to change a slave station S which may be in the transfer state into the stand-by state, as is shown in FIG. 7.

Furthermore, in the exemplary embodiment illustrated in FIGS. 6 and 7, there is advantageous provision, provided that only one slave station S can be, or is, connected to the communications system, to change said slave station S directly from the idle state into the transfer state by means of a sixth command CMD6. This single slave station S can be addressed by the master station M using the address which is permanently stored in the read-only memory REG2, without the slave station S having to be identified, accompanied by the allocation of a new address, using the commands CMD1, CMD2 and CMD3. In this way, in such a communications system where there is only one slave station S present, operation in the first operating mode, in which the first output circuits OC are activated, is dispensed with and the slave station S is operated exclusively in the second operating mode (corresponding to the transfer state). This results advantageously in a large time saving in terms of the actuation of the individual slave stations S.

The slave stations S may be, for example, data media in the form of cards and the master station M may be an appropriate write/read device or playback device. 

We claim:
 1. A communications system, comprising: a master station, at least one slave station, and at least one line connecting said master and slave stations; said at least one slave station including a first output circuit and a second output circuit each having an output commonly and directly connected to said at least one line without any active circuitry connected between said outputs of said first and second output circuits; said first output circuit of said at least one slave station, in a first operating mode of the system, serially outputting an identification code on said line; and in a second operating mode, said master station addressing said at least one slave station and said second output circuit of said slave station transmitting data different from the identification code to said master station through said line.
 2. The communications system according to claim 1, wherein said at least one slave station is one of a plurality of slave stations, each of said slave stations has a respective identification code, and in the first operating mode, said output circuits of said plurality of slave stations serially output their respective identification codes simultaneously on said line.
 3. The communications system according to claim 2, wherein, in the second operating mode, said master station addresses a selected one of said slave stations and only the respectively addressed slave station transmits data different from it's identification code to said master station.
 4. The system according to claim 1, which further comprises a clock generator in said master station and a clock line for transmitting a clock signal from said clock generator to said at least one slave station, said clock signal syrchronizing an operation of said master station and said slave stations; wherein a clock rate of said clock signal is lower in the first operating mode than in the second operating mode.
 5. The communications system according to claim 1, wherein said at least one slave station is one of a plurality of slave stations, and which further comprises connecting devices for connecting said slave stations, said connecting devices including detectors for detecting whether a respective slave station is connected and for transmitting a corresponding signal to said master station.
 6. A slave station for a communications system operable in two distinct operating modes, comprising: a first output circuit with an output serially outputting an identification code in a first operating mode of the communications system; and a second output circuit with an output outputting data different from the identification code in a second operating mode of the communications system; wherein said outputs of said first and second output circuits directly connected to one another without any active circuitry connected between said outputs of said first and second output circuit.
 7. The slave station according to claim 6, wherein said first output circuit assumes a state with a relatively higher impedance when a datum of a first logic state is output and a state with a relatively lower impedance when a datum of a second logic state is output.
 8. The slave station according to claim 7, wherein said second output circuit is a tristate output circuit.
 9. The slave station according to claim 6, wherein said second output circuit is a tristate output circuit. 